A wide variety of systems have been proposed for protecting digital content. Most such schemes encrypt the content to protect it against unauthorized use and copying while it is stored on media or sent over untrusted communication channels. Decryption algorithms and keys are then managed by trusted, tamper-resistant software or hardware modules, which are designed to enforce access control rules (which may be fixed or configurable) specifying how the content can be used.
Content protection schemes are generally customized for a particular playback environment. For example, anti-piracy systems in software-only streaming content players designed for personal computers lack the security benefits of tamper resistant hardware but can generally be upgraded without great difficulty (e.g., if the user uninstalls the player and downloads an updated version from the manufacturer web site). As a result, such systems may provide less robust security than hardware-based players, but the consequences of an attack are relatively small since upgraded security features can be deployed by modifying the content stream and requiring that users upgrade their software.
In contrast, protection methods embedded in consumer electronics hardware devices that play optical media are notoriously difficult to upgrade. Security challenges include the long lifetime of optical media (which prevents security upgrades that are not backward-compatible), the lack of a convenient and reliable way to deliver updates to players, and the lack of standardization between player implementations. These difficulties, combined with the long lifetime of playback devices and the consumer expectation that all new content will play on old players, make it extremely difficult to introduce security upgrades. As a consequence, most consumer electronics devices provide little or no real protection against copying, and the few content protection standards that are deployed in consumer electronics devices tend to be simple, rigid schemes that offer little flexibility and renewability. FIG. 1 diagrams a typical content protection system of the background art. Content player 100 includes software in nonvolatile program memory 105, which implements the player's security policies 110, decryption code 120, and player keys 130. This code and keys are used by processor 140 to validate whether the content read from media 150 is valid and, if so, to decrypt the content and supply the results to output interface 160. Examples of protection systems like the one shown in FIG. 1 include the copy control scheme used with digital audio tape, the content scrambling system (CSS) intended to protect DVD video, and the CPPM scheme proposed for protecting DVD audio.
A variety of different technologies are known in the background art:                Access control policies: A wide variety of access policies, and methods for specifying such policies, are known in the background art. For example, the software protection system of U.S. Pat. No. 4,658,093 to Hellman uses a straightforward authorization code issued by a publisher. In contrast, U.S. Pat. No. 5,982,891 to Ginter et al. describes a variety of very complex access rules involving a large number of participants. Standards for encoding access policies (both for use with content distribution and other applications) have also been proposed, such as PolicyMaker and the X.509 certificate format.        Anti-virus software: Methods for detecting and blocking known viruses, Trojan horses, and other malicious code are well known in the background art. These methods generally involve scanning for attributes of known viruses, such as known instruction sequences. These programs can work in a variety of ways, such as scanning files during start-up, scanning files on-the-fly, scanning programs as they execute, scanning memory, scanning new media, scanning network communications, etc.        Content protection systems and DRMs: A wide variety of content protection systems (which are also sometimes called Digital Rights Management (DRM) systems) have been proposed. DRM systems of the background art generally provide for content to be distributed in encrypted form, then supply decryption keys or perform decryption operations for legitimate purchasers. Many features have been proposed or included in commercial DRMs, including support for superdistribution (where encrypted content can be exchanged between users), pay-per-use billing (including off-line pay-per-use with reporting via a telephone line), variable billing rates (charging different amounts based on promotions, number or duration of uses, requested user operations, user history, etc.), protection for various data types (audio, video, text, software, etc.), support for various formats, and support for various playback device types (portable, set-top, computer-based with hardware assistance, software-only, etc.)        Copy protection: Methods for copy protecting personal computer software are known and are widely deployed for some kinds of software such as computer games. These methods often involve binding a software program to physical media that are designed to be difficult to copy (e.g., by intentionally incorporating errors or nonstandard formatting that are difficult to replicate). Other copy protection systems involve securing the installation process, e.g. by requiring that users obtain an authorization code from a server. In some cases, copy protection features are designed into a system. In others cases (including copy protection systems used for computer software, videocassette tapes, and audio CDs), copy protection is implemented by producing media with nonstandard encoding that allows playback on most players but will confuse most attempts to copy the media. A major design challenge for copy protection systems is to minimize the impact on legitimate users (i.e., obtain high playability and user acceptance) while preventing undesirable actions as effectively as possible (i.e., obtaining good security).        Cryptographic functions: A wide variety of basic cryptographic functions are known, including block ciphers, hash functions, digital signature systems (and other public key systems), key management systems, etc. For more information about basic cryptography, see Applied Cryptography by Bruce Schneier.        Cryptographic oracles: Using block ciphers or other cryptographic functions, it is possible to construct “cryptographic oracles” which apply a secret cryptographic transformation to arbitrary externally-supplied input messages and return the results. Cryptographic oracles can be constructed so that it is computationally infeasible for an attacker who knows the oracle's algorithms and protocols to determine the oracle's keys. In addition, because the number of possible inputs to an oracle can be extremely large (e.g., 2256 for an oracle constructed from a 256-bit block cipher), it is not feasible for an attacker to anticipate or pre-compute the responses to random queries.        Interpreters, emulators, and virtual machines: A variety of interpreted computer languages are known in the background. Some interpreted languages, such as Java, require a compilation process to convert source code into an executable or interpretable form. In contrast, most BASIC interpreters operate directly on the source code. Some interpreters allow self-modifying code, while others do not. Technology for implementing interpreters and for emulating assembly languages is also known in the background art. For example, sophisticated emulators such as Virtual PC and SoftWindows can run programs designed for Microsoft Windows on Apple Mac computers. Virtual machine (VM) designs, such as those used for Java and JavaCard, are known, and it is also known that VMs can interact with native code on the computer, or call other VM functions in different memory spaces. (Many Java implementations provide these capabilities.) Interpreted languages are commonly used for applications or where cross-platform compatibility is required, such as for creating processor-independent device driver formats. (See, for example, Writing FCode 2.x Programs, Sun Microsystems, 1993, page 5.)        Key management: A wide variety of methods for assigning and managing cryptographic keys have been proposed. It is known that devices can have device-specific keys, group keys, public keys, private keys, certificates, etc. Keys can be assigned to individual devices, to selected groups of devices (e.g. as described in U.S. Pat. No. 5,592,552 to Fiat), to all devices, etc. Devices can contain a variety of keys of different types, including symmetric keys, public keys (e.g., to verify certificates and digital signatures) and asymmetric private keys.        Media: Media technologies are known that can offer tremendous storage capacity, low manufacturing cost, and good durability. Examples of current media technologies include optical discs (CD, DVD, etc.), magnetic media, flash memory, and ROMs. Newer technologies, such as holographic memories, are also being developed. It is known that a single piece of media can include data of many different types. For example, a compact disc can contain standard Red Book audio tracks as well as a data session for use on personal computers (e.g., containing software, compressed bonus tracks, images, videos, lyrics, etc.) Compact discs for use for use in personal computers can contain both encrypted content as well as the playback software required to play the content.        Network communication: Sophisticated data networks, including the Internet, are known. These networks can provide flexible, reliable, high-bandwidth data communication. Although networks with a physical connection usually provide higher bandwidth, wireless communication channels are also popular.        Renewable security: In some cases, it is not practical to produce a security system that is guaranteed to be able to prevent all possible attacks. As a result, it is desirable that it be possible to renew security after an attack, e.g. by discontinuing the use of any compromised keys and correcting the vulnerability. Although renewable security is desirable, many deployed and proposed systems lack any effective recovery mechanism for many kinds of attacks.        Sandboxing: Sandboxing involves executing software programs in a controlled environment where the program is unable to access any operations that could damage the system. The Java “virtual machine” supports sandboxing so that untrusted applets (such as those downloaded over the Internet) can be executed.        Security modules: Many security systems employ removable security modules so that the security upgrades can be performed without the difficulty or expense of replacing other portions of the system. For example, removable security modules are used in many pay television systems.        Software updates: Secure software updates can be performed by receiving a proposed software update, verifying a digital signature or message authentication code validating the update, then (if the signature is valid) performing the update. For example, it is known that digital audio players can receive code updates, verify digital signatures or message authentication codes on the updates, and (if valid) update their code. Methods for ensuring that updates are applied in the correct order (e.g., using sequence counters) and for recovering from failed or unsuccessful updates (e.g., by reverting to previous software versions or by activating special recovery code) are also known. It is also known that software updates can be delivered via virtually a wide variety of distribution mechanisms, such as the Internet, optical media, ROM cartridges, etc. Software updates have been used to prevent pay television piracy by distributing code updates with the signal to descramblers, which apply and successfully execute the new code to compute the correct decryption key for the next video segment. These updates are commonly used to prevent unauthorized viewing by disabling or even destroying unauthorized descramblers.        Stepanograph: Steganography involves hiding information in data. For example, it is known that encrypted data can be placed in the least-significant bits of an image or sound recording. An attacker who obtains this image or recording but does not know the decryption key cannot even determine whether there is any hidden data because low-order bits often appear random and ciphertext produced by a strong encryption algorithm cannot be distinguished from random data without the key.        Tamper resistance: Many methods are known for designing and constructing devices that are resistant to attack. Tamper resistant hardware is commonly used in systems where it is desirable to prevent attackers from reverse engineering devices or extracting keys from cryptographic modules. For example, Wave Systems markets a tamper-resistant microprocessor-based integrated circuit product called “Embassy” which can be integrated with content players or general-purpose computers and is advertised for use in securing the distribution of digital content. Methods for implementing tamper resistant software have also been proposed (see, for example, U.S. Pat. No. 5,892,899 to Aucsmith et al.).        Traitor Tracing: Traitor tracing schemes have been proposed to identify the source of compromises or attacks, typically by tracing keys used in unauthorized devices back to a customer particular or compromised device.        Watermarking: Watermarks are signals embedded in content that can be detected by a specialized detector but do not affect (or minimally affect) human perception of the content when played. Watermarks embedded in pictures, sound recordings, and images have been used by copyright holders to indicate that copying is not authorized. “Robust” watermarks are known that can withstand conversions between formats (including re-recording from analog outputs) and provide varying degrees of security against attacks attempting to remove the watermark. In contrast, “fragile” watermarks have little or no ability to withstand format conversions but are easier to design and can carry more information.        
Although no anti-piracy system can completely prevent all possible attacks, systems of the background art fail to provide practical solutions to solvable problems such as casual piracy using digital-to-digital copying or high-speed ripping of protected formats to unprotected formats. Significant limitations of many systems of the background art include, without limitation:                Reliance on global secrets: Many protection systems require that cryptographic algorithms, keys, and other information needed for decoding be kept secret. As a result, the decoding process cannot be documented in open standards documents without compromising the security of the system. Also, if a large number of implementations are available, attackers can potentially break the entire scheme by attacking the weakest implementation. (Such an attack recently occurred with the DVD video protection system.) While such systems are useful in closed single-vendor environments, they cannot be standardized and do not provide effective long-term security.        Lack of standardization: Content publishers have already committed to a variety of data formats and decryption algorithms that are incompatible. Different content protection systems enable different business models, and publishers who have committed to one model are likely to oppose any security system that requires a different model.        Incompatibility with product types: Many security features cannot be integrated with all product types. For example, downloadable software-only players for personal computers cannot include tamper-resistant hardware. Similarly, frequent software updates are difficult to deliver to players lacking Internet connectivity.        User interface: Many proposals involve complex user interfaces. Security should be invisible to honest users. Users are likely to reject schemes that require explicit user involvement (e.g., to obtain or enter authorization codes). In general, consumer electronics devices such as car stereos and video disc players must be easy-to-use, since many users must be satisfied even if they do not read documentation, are intimidated by technology, have poor eyesight or other handicaps, or lack fluency in the languages supported by the player.        Legal challenges: Some security systems require cooperation between competitors. Such cooperation can be illegal due to antitrust regulations.        Lack of manufacturer benefit: Manufacturers will oppose security features that increase player cost, time-to-market, prevent the inclusion of legitimate features, or otherwise make their products less effective or desirable. Although advances in semiconductor technology are decreasing the cost required to implement security systems, effective tamper-resistant hardware remains difficult and expensive to design and produce. As a result, content protection systems that rely on manufacturers to produce good implementations will fail unless they provide a real marketplace advantage to manufacturers whose offerings are more secure.        Indefinite security policies: Effective security systems must specify rules or other decision-making procedures for determining whether to allow or prevent user-requested specific actions. In many systems, these rules or procedures are not well specified.        Inflexible security policies: It is desirable for content protection systems to have the flexibility to support different models for different publishers, content types, jurisdictions, playback environments, etc. Systems should offer the necessary flexibility without becoming too complex.        Weak long-term security: Security systems must be robust and flexible enough to remain effective for a long time. Few content protection systems of the background art could last more than a few years as part of a high-profile format, while a popular format can last for more than 30 years.        Untraceability of attacks: If attacks do occur, systems should be able to identify the source of the attack so that the compromised (or misused) device can be revoked and so that criminals can be prosecuted.        